In the separation of components of a biological or chemical sample, both torsional vibrations and lateral vibrations within the centrifuge can result in equipment failures. Torsional vibrations are also referred to as torque ripple, and are manifested as changes in the angular velocity of a motor armature compared to a sample-containing rotor. Torsional vibrations generate stresses that alone may be sufficient to jeopardize a drive shaft of the centrifuge. However, the torsional vibrations typically induce some lateral vibration, so that the combination of torsional stresses and torsionally induced lateral stresses further reduces the safety factor of the system.
Regarding lateral vibration, at the critical speed of the rotating system of the centrifuge, the rotating system becomes dynamically unstable. As a result, large lateral amplitudes create "whirl," with the sample containing rotor bending the drive shaft. This phenomenon occurs when the angular velocity excites the natural frequency of lateral vibrations. A centrifuge is particularly susceptible to shaft failure when these lateral vibrations are combined with torsional vibrations.
It is known to increase the safety factor of a centrifuge by designing a drive mount that provides both vibration isolation and damping between a drive motor and a frame of the centrifuge. U.S. Pat. No. 4,193,536 to Kubota describes vibration-isolating supports positioned between a mounting plate and a bottom plate, with the drive motor being supported in such a manner as to minimize transmission of the vibration of the motor to an outer frame. The resulting foundation flexibility and damping increases the critical speed, i.e. whirl threshold speed, of the rotary system.
Improvements in damping torsional vibrations can be achieved by various techniques. The design of the coupling assembly of a drive motor to a drive shaft will affect the extent to which torsional stress and torsionally induced lateral stresses are generated during operation of a centrifuge. Thus, damping structure within the motor-to-shaft assembly is common. On the other hand, damping structure is typically not included within the assembly that transmits torque from the drive shaft to the sample-containing rotor. U.S. Pat. No. 4,205,261 to Franklin, which is assigned to the assignee of the present invention, describes the drive shaft as being connected directly to a hub, which is connected directly to the rotor. A difficulty with conventional damping techniques being applied at the shaft-to-rotor transmission assembly is that torsional damping is typically accompanied by damping of lateral vibrations. A key to maintaining the factor of safety for the centrifuge is the maintenance of a ratio of lateral damping at the foundation system to lateral damping at the rotary system. Thus, unless the foundation system is adjusted, torsional damping at the shaft-to-rotor assembly will lead to lateral damping that renders the centrifuge more susceptible to high speed whirl that can cause shaft failure.
It is an object of the present invention to provide an assembly for connecting a drive shaft to a driven rotor-supporting hub, wherein the assembly is selective with respect to damping torsional vibrations.